Oh heck, so you were quoting from a document that shows, 501500. This provides now three Philatelic order item numbers, 561500, 501500 and 501540 for what has been established as being produced in two printings.
That said, the 561500 stamps are the one issued for the First Day of Issue of April 17, 2001. The stamps sold under the item number 501540 are the April 2003 reprints per the USPS. But there is no clear answer to which, 1st or reprint, the 501500 are unless you know the date the 501500 were purchased AND we'd have to know if the 1st Printing and the Reprint were ever offer for sale at the same time. UGH.
Comment to hoarding: There are reasons to keep documentation long after it is out of date because it may need to be accessed as a reference. For domestic harmony, my reference hoard was sent off to a philatelic library while I trust most other copies hit the recycle bind or bird cage floor no long after arrival.
Edited to add more C137 inventory item numbers.
FDC 561561

Half pane 561510

